The Thymus Gland PDF

L6 : Dr. Hanan Al-Taee Dept. of physiology Objectives:  Definition &location.  Function of thymus gland.  Dysfunction of thymus gland. Thymus gland: The thymus gland is a specialized primary...

L6 : Dr. Hanan Al-Taee Dept. of physiology Objectives:  Definition &location.  Function of thymus gland.  Dysfunction of thymus gland. Thymus gland: The thymus gland is a specialized primary lymphoid organ of the immune system; it is located in the upper front part of the chest, in the anterior superior mediastinum, behind the sternum, and in front of the heart. It is made up of two lobes, each consisting of a central medulla and an outer cortex, surrounded by a capsule. It is largest and most active during the neonatal and pre-adolescent periods. At birth, it is about 4–6 cm long, 2.5–5 cm wide and about 1 cm thick. It increases in size until puberty, following which it decreases in size by involution and gradually replaced by fibrous tissue. Nevertheless, some T cell development continues throughout adult life. Within the thymus, thymus cell lymphocytes or T cells mature. T cells are an important part of the immune system providing cell-mediated immunity. T cells begin as hematopoietic precursors from the bone-marrow, and migrate to the thymus, where they are referred to as thymocytes; they are attracted to the thymus gland by hormones and cytokines secreted by cells within the thymus, including thymulin, thymopoietin, and thymosins. The maturation process involves ensuring that T lymphocyte cells react against antigens ("positive selection"), but that they do not react against antigens found on body tissue ("negative selection"). Most T cell receptors bind to the major histocompatibility complex on cells of the body. The MHC presents an antigen to the T cell receptor, which becomes active if this matches the specific T cell receptor. Positive selection occurs in the cortex and negative selection occurs in the medulla of the thymus. After this process T cells that have survived leave the thymus, regulated by sphingosine-1-phosphate. Further maturation occurs in the peripheral lymphoid organs. Congenital problems with the development of the thymus can lead to immunodeficiency Abnormalities of the thymus can result in a decreased number of T cells and autoimmune diseases such as autoimmune polyendocrine syndrome type 1 and myasthenia gravis.